The disease manifestations in HTLV-I (human T-cell lymphotropic virus)-infected individuals are highly diverse, ranging from malignancies to autoimmune-like diseases. Since no differences in virus structure or function have been found in isolates from individuals with the various diseases, differences in disease manifestation may reside in the cell targeted for infection and/or the host's response to infection. The cell infected in vivo with HTLV-I or HTLV-II (a virus similar to HTLV-I; however, with no disease associations) was identified in subpopulations of peripheral blood lymphocytes separated on the basis of cell surface markers. The CD4+ and CD8+ cells were infected with virus in HTLV-I-positive individuals; however, virus expression was found to be predominant in CD4+ cells. In HTLV-II-infected individuals, provirus and expressed virus were found in CD+8 cells and occasionally in CD4+ cells. A disease manifestation of HTLV-I infection in childhood is infectious dermatitis. Distribution of peripheral blood T-cell subsets with and without activation markers were compared in HTLV-I-infected and noninfected children. Increased numbers of activated CD4+ cells were found in infected children that were positively correlated with duration of infection.